Tunable-frequency oscillators are used for widely different purposes. Inter alia, they provide the clock signal for digital circuits in mobile radio devices. The output frequency of the oscillator varies because of external influences, such as temperature changes. It is thus necessary to retune the oscillator again, and to change the output frequency.
In the case of voltage controlled oscillators (VCOs), this is achieved by application of a continuous voltage to a capacitance which is coupled to the resonant circuit of the oscillator. The capacitance in this case determines the resonant frequency of the oscillator. Variation of the capacitance thus also results in the output frequency being changed, because of the voltage change.
In contrast to this, in the case of a digitally controlled oscillator (DCO), discrete-value capacitances are added to or disconnected from the resonant circuit of the oscillator. The variation of the capacitance in steps thus produces a frequency change in the form of individual sudden changes at the output of the digitally tunable oscillator. This discrete-value frequency change results in a sudden phase change in the output frequency.
In modern communication systems, the various circuits use the same reference frequency and the same reference oscillator. For example, in a mobile radio, the GSM system part as well as the Bluetooth transceiver use the clock signal from the same digitally tunable oscillator. The GSM system is very sensitive to fluctuations in the frequency, and now produces a control signal in order to set a new clock frequency for the digitally tunable oscillator. This results in a sudden phase change being produced in the output signal from the oscillator, which is also evident in the clock signal. If the Bluetooth transceiver is in a receiver mode at the same time, the transceiver may possibly lose received data as a result of the sudden phase change in the clock signal, or may receive this only incorrectly.
Since, in general, all digital and analog circuits react sensitively to phase changes in the clock signal, it is desirable to minimize the sudden phase change. The voltage controlled oscillators therefore use low-pass filters which filter the programmed voltage change and thus produce only a very slow and small phase change. The disadvantage of this filter method is, however, the long time period which is required for a frequency change. In consequence, the current drawn is also increased and the life, in particular of mobile radios, falls. In the case of a digitally tunable oscillator in which a frequency change is carried out by means of a discrete sudden change in the capacitance, such filtering is also possible, but the discrete sudden change in the capacitance cannot be completely avoided.